1. Field of the Invention
The present invention relates to a nonaqueous electrolyte secondary battery and a method of manufacturing the same.
2. Description of Related Art
A nonaqueous electrolyte secondary battery such as a lithium ion secondary battery (lithium secondary battery) has a lighter weight and higher energy density than those of existing batteries and thus, recently, has been used as a power supply for driving a vehicle. In particular, a light-weight lithium ion secondary battery capable of obtaining high energy density is preferably used as a high-output power supply for driving a vehicle such as an electric vehicle (EV), a hybrid vehicle (HV), or a plug-in hybrid vehicle (PHV). Typical examples of such a nonaqueous electrolyte secondary battery include a nonaqueous electrolyte secondary battery including a wound electrode body. The wound electrode body is formed by laminating an elongated sheet-shaped positive electrode current collector foil, an elongated sheet-shaped negative electrode current collector foil, and an elongated sheet-shaped separator and winding the obtained laminate in a sheet longitudinal direction. The elongated sheet-shaped positive electrode current collector foil has a surface, on which a positive electrode active material layer containing a positive electrode active material is formed, and is typically aluminum foil. The elongated sheet-shaped negative electrode current collector foil has a surface, on which a negative electrode active material layer containing a negative electrode active material is formed, and is typically copper foil. The nonaqueous electrolyte secondary battery including the wound electrode body is constructed by accommodating the wound electrode body and a nonaqueous electrolytic solution in a predetermined battery case. Here, this nonaqueous electrolytic solution is mainly impregnated into the inside of the wound electrode body (that is, a gap between the positive and negative electrodes and the separator) from opposite end portions of the wound electrode body in a winding axial direction (that is, a width direction perpendicular to the sheet longitudinal direction of the elongated sheet-shaped positive and negative electrode current collector foils constituting the wound electrode body). In order to adjust the constructed nonaqueous electrolyte secondary battery to be in an actually usable state, the nonaqueous electrolyte secondary battery is initially charged under appropriate conditions.
During the initial charging, a portion of the nonaqueous electrolytic solution is reduced and decomposed on the negative electrode to form a film, which is also called a solid electrolyte interface (SEI), on a surface of the negative electrode active material. By the film coating the negative electrode active material layer, the negative electrode is stabilized, and the subsequent decomposition of the nonaqueous electrolytic solution is suppressed. However, the decomposition of the nonaqueous electrolytic solution contributes to the irreversible capacity, which causes a decrease in battery capacity. Therefore, a technique of adding an additive (hereinafter, referred to as “film forming agent), which is decomposed at a decomposition potential or lower of the nonaqueous electrolytic solution so as to form a film on the surface of the negative electrode active material layer, to the nonaqueous electrolytic solution in advance has been widely used. For example, Japanese Patent Application Publication No. 2005-259592 (JP 2005-259592 A) discloses a nonaqueous electrolyte secondary battery in which a nonaqueous electrolytic solution contains lithium bis(oxalato)borate (Li[B(C2O4)2]; hereinafter, referred to as “LiBOB”) as a film forming agent.